1. Field of the Invention
This invention relates generally to a fuel cell system that includes a process for minimizing corrosion in the cathode side of a fuel cell stack and, more particularly, to a fuel cell system that includes a process for minimizing corrosion in the cathode side of a fuel cell stack, where the process includes combining a stack electrical shorting technique and a cathode re-circulation technique at system start-up and shut-down.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode, typically by a catalyst, to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons, typically by a catalyst, in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have four hundred stacked fuel cells. The fuel cell stack receives a cathode reactant gas as a flow of air, typically forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
When a fuel cell system is shut down, un-reacted hydrogen gas remains in the anode side of the fuel cell stack. This hydrogen gas is able to diffuse through or cross over the membrane and react with the oxygen in the cathode side. As the hydrogen gas diffuses to the cathode side, the total pressure on the anode side of the stack is reduced below ambient pressure. This pressure differential draws air from ambient into the anode side of the stack. When the air enters the anode side of the stack it generates an air/hydrogen front that creates a short circuit in the anode side, resulting in a lateral flow of hydrogen ions from the hydrogen flooded portion of the anode side to the air-flooded portion of the anode side. This high ion current combined with the high lateral ionic resistance of the membrane produces a significant lateral potential drop (˜0.5 V) across the membrane. This produces a local high potential between the cathode side opposite the air-filled portion of the anode side and adjacent to the electrolyte that drives rapid carbon corrosion, and causes the carbon layer to get thinner. This decreases the support for the catalyst particles, which decreases the performance of the fuel cell.
It is known in the art to purge the hydrogen gas out of the anode side of the fuel cell stack at system shut-down by forcing air from the compressor into the anode side at high pressure. The air purge also creates an air/hydrogen front that causes the cathode carbon corrosion, as discussed above. Thus, it is desirable to reduce the air/hydrogen front residence time to be as short as possible, where the front residence time is defined as the anode flow channel volume divided by the air purge flow rate. Higher purge rates will decrease the front residence time for a fixed anode flow channel volume.
It is also known in the art to provide cathode re-circulation to reduce cathode corrosion at system shut-down. Particularly, it is known to pump a mixture of air and a small amount of hydrogen through the cathode side of the stack at system shut-down so that the hydrogen and oxygen combine in the cathode side to reduce the amount of oxygen, and thus the potential that causes the carbon corrosion.
It is also known to short circuit the stack with a suitable resistor at system shut-down to reduce the amount of oxygen on the cathode side of the stack, and thus cathode side corrosion. It has been shown that these two techniques do provide mitigation of carbon corrosion on the cathode side of the stack. However, improvements can be made.